The present invention relates to a method for determining the detaching angle and/or the detaching profile of a paper web, in which the paper web travels through sequential guide devices and detaches from a guide device, in such a way that the paper web deviates, from the detaching point onwards, from the tangential angle of the detaching point and that the detaching point forms a detaching profile in the transverse direction of the paper web.
In conventional paper machines, there are some particular points, at which the paper web is detached from the surface of a guide device without support. Such a point is, for example, the removal of the web from the centre roll in the press section, when the web does not move tangentially to the next roll, but is sucked onto the centre roll, until there is sufficient gravity to deflect the web in the direction of the next roll. The point of detachment is determined by the properties of the web, in particular, its moisture content. In addition, dirt on the centre roll will affect the detachment of the paper from it. Detachment in the cross-machine direction does not take place exactly simultaneously, instead the detachment point forms a particular profile. This detaching profile correlates strongly with the moisture profile of the paper web, so that it can be exploited when determining the moisture profile. On the other hand, the aforesaid guide surface, for example, the dirtying of the roll, can greatly alter the detaching profile.
Previously, the detachment point has been measured using a laser scanner, which has determined the distance of the detachment point from a fixed point.
The present invention is intended to create a new kind of method for determining the detaching angle and/or detaching profile of a paper web. The addition of suitable lighting allows the detaching profile to be seen and thus to be recorded by video photography. Known pattern recognition devices can be used to define the boundary between the light and dark areas as a geometrical line.
According to one embodiment, the paper web is illuminated from below and photographed from above, when the through-lighting makes the detached paper web considerably lighter and the attached paper web appears as extremely dark.
According to another embodiment, the average location of the detachment point is illuminated with a laser photo-electric trip device and is photographed at an angle using a video camera, in which case the changes in the detachment point are clearly visible as movements of the image point in the video image.
In the following, the invention is described by reference to the accompanying illustrations, which show some embodiments according to the invention.